It is a recent tendency to impart greater accuracy and higher function to semiconductor elements. Accordingly, the structure of such semiconductor elements has become more and more precise and complicated. Accordingly, to meet such requirements, a film of a uniform thickness and composition is formed on the surface of a substrate, and then a structure of elements having complicated unevenness on the surface is realized utilizing a highly accurate lithography technique. Recently, for the purpose of simplifying the complicated processing steps for fabricating these elements, it has been tried to control the thickness of a part of a film to be formed on the surface of a substrate during the formation of the film. For instance, in Applied Physics Letters, Vol. 47, 1985, p. 95, there is disclosed a technique which comprises irradiating a substrate with light from an argon laser while forming a GaAs film according to metalorganic chemical vapor desposition method (hereunder referred to as "MOCVD"), to thus form a film only on the portions irradiated with the argon laser rays. The reason why the film is selectively grown on the irradiation portions is that the growth of the film is enhanced by the decomposition of an organometal as a starting material by irradiating it with light. It has been reported that a low pressure mercury lamp, an excimer laser or the like is also effective as a light source other than the argon laser.
As the accuracy and the function of semiconductor elements such as those for optoelectronics become higher, the complexity of the process for fabricating the semiconductor elements has steadily increased.
Recently, there has been developed a metalorganic molecular beam epitaxial growth technique as a novel film growth method. The method is characterized by the use of such a molecular beam.
The molecular beam will now be explained below. The molecular beam refers to the condition that a molecule discharged from a source of the molecular beam reaches a substrate without causing any collisions with molecules remaining in a vacuum chamber. To realize such a condition, the inside of a deposition chamber should be maintained at a high vacuum. In general, the distance between the source of a molecular beam and a substrate ranges from 10 to 20 cm. The mean distance that a molecule proceeds between two consecutive collisions, i.e., the mean free path L can be expressed by the following equation as a function of an internal pressure p of the vacuum chamber: L (cm)=10.sup.-2 /p (Torr). Therefore, to establish the mean free path L of 10 cm, it is necessary to maintain the inside of the growth chamber at a pressure of not more than 10.sup.-3 Torr.
For the purpose of simplifying processes in fabricating semiconductor devices according to this method, there have been proposed not only structures of such devices but also processes for fabricating semiconductor films. For instance, in Applied Physics Letters, Vol. 52, No. 13, 1988 (Mar. 28), p. 1065, there is disclosed a technique for forming a semiconductor film by partially irradiating a semiconductor substrate placed in a metalorganic molecular beam epitaxial (hereunder referred to as "MOMBE") system with, for instance, excimer laser rays during the fabrication of a semiconductor film to thus selectively form such a semiconductor film on a part of the semiconductor substrate which is irradiated with the laser rays.
However, the excimer laser principally suffers from the following two disadvantages. First, since the wavelength of the laser agrees with that of the light absorbed by an organometal, the organometal is decomposed not only on the substrate but also in the atmosphere. Second, since the excimer laser is a pulse oscillating laser, the energy of the pulse is extremely high. This leads to substantial increase in the temperature of the substrate in proportion to the irradiation of the substrate with the laser. It is difficult to obtain patterns correctly corresponding to the distribution of light, due to these drawbacks. As will be seen from the data listed in Table I, an argon laser is preferable for obtaining such patterns exactly corresponding to the distribution of light.
TABLE I ______________________________________ Argon Laser Excimer Laser ______________________________________ Form of light continuous light pulse light Wavelength 0.35-0.51 .mu.m 0.19-0.25 .mu.m Power density 1 10.sup.5 (relative ratio) Temperature rise 20.degree. C. about 500.degree. C. or more of substrate Direct decomposition not observed observed of organometal ______________________________________
In the conventional selective growth by irradiation of light, the MOCVD method for growing a semiconductor film has been exclusively used. This presents a disadvantage that a fine pattern cannot be obtained. The causes of the disadvantage are that (a) in the MOCVD method, the internal pressure of the growth chamber is very high; a film is grown in a hydrogen gas atmosphere having a pressure of several tens to 760 Torr; therefore, a flow of an organometal is induced on a substrate, which results in the formation of a selective film having a smooth shape: and (b) since a growth chamber or a reaction tube is a cylindrical glass tube, it is impossible to project a fine light interference pattern on a substrate; and the possibility of such selective growth also depends on the conductivity type of substrates used; e.g., the selective growth takes place on n-type substrates while a film cannot be selectively grown on a semi-insulating substrate. This presents a substantial restriction when the selective growth is applied to the formation of devices.
Furthermore, in the conventional selective growth by the irradiation of light, the light is irradiated at a low substrate temperature at which the growth of a film is enhanced with the rise of the substrate temperature. This is because a part of an organometal supplied onto the substrate remains undecomposed at a low temperature, and it is used to increase the growth rate of the film through decomposing by the irradiation with light. As has been well known, the quality of a film is deteriorated as the growth temperature decreases. For this reason, films selectively grown according to a conventional method are inferior in their quality, and thus no film has ever been used for fabricating devices.
The MOMBE method makes it possible to eliminate all of the foregoing disadvantages associated with the MOCVD method. The characteristics of these methods are summarized in the following Table II. As will be seen from Table II, the MOMBE method makes it possible to irradiate a substrate with a fine light pattern.
TABLE II ______________________________________ MOCVD MOMBE ______________________________________ Apparatus cylindrical super-high vacuum quartz tube stainless chamber Pressure during 10 Torr or more 10.sup.-3 Torr or less film growth Method for growing supplying a supplying a films (FIGS. 1, 2) starting material starting material with a gas stream as molecular beam and thermally- and thermally- decomposing it decomposing it above the substrate on the substrate Material of Group III organometal organometal Material of Group V hydride thermally- decomposed hydride or metal Incidence of fine difficult easy light patterns ______________________________________
However, when irradiating a substrate within an MOMBE apparatus with light from a laser light source, the MOMBE system and an optical system including a source of laser are separately or independently placed. For this reason, the relative position of the MOMBE system with respect to the optical system varies owing to vibrations in the MOMBE system or caused by other apparatuses, and the aberration of the positions will result. Therefore, the projection of a fine beam or a fine pattern on the substrate is impossible, and further, it is difficult to converge the beam on the substrate sharply with the use of a lens, and contamination of a window is observed during growing a film. These problems will hereunder be discussed in more detail.
Upon selectively growing a fine pattern according to this method for forming a thin film of a semiconductor, it is important to maintain the relative position constant between the growth apparatus and an optical system including lenses and mirrors. To this end, it is conceived, for instance, that the optical system and the growth apparatus are mounted on a vibration proof base.
However, these laser source, optical system and growth apparatus mounted on the vibration proof base are connected to the exterior for the necessity of supplying an electric current, cooling and evacuation. The external vibrations thus exert influence on these laser source, optical system and growth apparatus mounted on the vibration proof base through power supply cords, a cooling water tube, a piping for the vacuum pumping system or the like.
Furthermore, in the conventional apparatus for growing semiconductor films, a substrate-rotating-and-heating portion in the growth system is supported at one side by a supporting bar having only a single bar as a principal component. Thus, the external vibrations transmitted to the vibration proof base easily results in the vibration of the substrate-rotating-and-heating portion, which in turn makes it difficult to maintain the distance constant between the optical system and the substrate within the growth system and to thus selectively grow a fine pattern on the substrate. For instance, in the case where a diffraction grating for a DFB laser is grown through the interference between two laser beams, it is necessary to grow a fine pattern on the order of 0.3 to 0.5 .mu.m, and hence, the amplitude of vibrations of a substrate holder for supporting the substrate and the substrate-rotating-and-heating portion must be sufficiently small compared with the size of the fine pattern. For this reason, the conventional apparatuses cannot grow such a fine pattern.
To obtain a desired pattern of a semiconductor thin film according to the MOMBE method for forming a thin film of a semiconductor, it is of primary importance for a laser beam for irradiating a semiconductor substrate to be controlled and converged to a fine beam. In this respect, it has been known that the diameter of the beam waist (the portion at which the laser rays are converged most finely through a lens) is proportional to the focal length of the lens used. For this reason, it is preferred to use a lens of the shortest possible focal length, and it is also desirable to place the lens as close as possible to the semiconductor substrate. In the conventional MOMBE system, however, the optical system such as a lens is placed outside a window for introducing light into the system and, therefore, it is impossible to sharply converge the laser beam. For instance, when the distance between the substrate and the window ranges from 150 to 600 mm, a lens is placed between the window and the substrate, and the substrate is irradiated with a laser beam having a diameter of about 1.5 mm, the laser beam can only be converged to a diameter of 90 to 400 .mu.m on the surface of the substrate.
Moreover, in the method for forming a thin film of a semiconductor, it is necessary to irradiate a substrate with a laser beam through a window provided on the growth system in order to obtain a desired pattern of a semiconductor thin film. This type of apparatus for growing thin films, such as a molecular beam epitaxial (MBE) system, or an MOMBE apparatus, has a shutter capable of opening and closing the window to prevent the contamination of the window due to the deposition of a semiconductor material by opening the shutter only at a desired time during the film growth, or just before and after the film growth. However, the shutter must be opened for a long period to irradiate the substrate with the laser beam during the film growth. As a result, the window becomes opaque after several cycles of film growth operations, and thus the laser rays cannot be incident upon the substrate. This requires the opening the thin film growth chamber, and the detaching of the window for washing about once a week. Thus, the thin film growth system is often exposed to the air and therefore, the quality of the resulting thin films is insufficient for use in making semiconductor devices.